Conventionally, “point-to-multi-point” access optical communication systems called passive optical network (PON) systems have been widely employed as systems capable of realizing public networks using optical fibers.
A PON system includes one set of optical line terminal (OLT) which is a station-side apparatus, and optical network units (ONUs) corresponding to a plurality of subscriber terminal apparatuses and connected via an optical star coupler to the OLT. Currently, the PON systems have been actively introduced in order to realize broadband networks from the following merits: since a major portion of optical fibers corresponding to transmission paths can be commonly used with the OLT for a large number of ONUs, lower operation cost can be expected; an optical star coupler corresponding to a passive component has no need for power feeding and can be easily set outdoors; and also reliability thereof is high.
For instance, in Gigabit Ethernet (registered trademark)-passive optical network (GE-PON) whose transmission speed is 1.25 Gbit/s and which is standardized in IEEE 802.3ah, in a downlink from an OLT to ONUs, there is employed a broadcast communication system using an optical wavelength 1.49 μm band, and the respective ONUs derive only data of the allocated time slots. On the other hand, in an uplink from the respective ONUs to the OLT, an optical wavelength 1.31 μm band is employed, and a time division multiplexing communication system is employed by which transmission timing is controlled in order that data of the respective ONUs do not collide with each other.
In communication of the uplink direction of the above-mentioned PON system, because the respective ONUs are located at different distances from the optical star coupler, reception levels of the respective ONUs in the OLT are different for every reception packet. As a result, a wide dynamic range characteristic capable of stably reproducing packets which have light receiving levels different from each other is required for a receiving circuit of the OLT. As a consequence, generally speaking, there is provided an automatic gain control (AGC) circuit in a receiving circuit of an OLT, which changes a conversion gain in response to a light receiving level.
Various types of AGC circuits have been proposed, and there is an AGC type circuit in which a feedback resistance value of a preamplifier is switched in a stepwise manner in response to a light receiving level (refer to, for example, Patent Document 1). Also, there is another AGC type circuit in which a feedback resistance value of a preamplifier is switched in an analog mode in response to a light receiving level (refer to, for example, Patent Document 2). Also, there is a further AGC type circuit having such a function that an amplification factor is changed in an analog mode, and a time constant of an AGC loop is also changed at the same time (refer to, for instance, Patent Document 3).
Patent Document 1: JP 2000-151290A
Patent Document 2: JP 07-38342 A
Patent Document 3: JP 06-91481 A